FIG. 6 shows a system of a conventional PWM cycloconverter. In FIG. 6, 1 denotes a three-phase AC power source, 2 denotes a three-phase AC reactor, 3 denotes a three-phase AC capacitor, 4 denotes a bi-directional switch group, 5 denotes a motor functioning as a load, 6 denotes a three-phase AC reactor, 7 denotes a PWM converter for converting AC into DC, 14 denotes a smoothing capacitor, which is a DC voltage means, 91-3 denote a detection current signal of the three-phase AC power source, 10 denotes a CT for detecting electric current, which is a current detection means for detecting electric current of the three-phase AC power source, 11 is a CT for detecting electric current, which is a current detection means, 121-3 is a detection current signal of the CT for detecting electric current and 151-9 denote snubber circuits provided on respective bi-directional switches of the bi-directional switch group 4.
In FIG. 6 showing a conventional embodiment, a small-sized PWM converter is used for keep down resonance voltage of an input filter, similarly to the invention. In FIG. 6, however, an output of the PWM converter is connected after (on a secondary side of) the filter for the purpose of keeping down voltage. Accordingly, applying electric current more than the quantity equal to correction of a resonance component adversely causes large variation in condenser voltage, and therefore, quantity of correction is limited. The conventional PWM converter has such a demerit. Therefore, an effect is limited especially in a system where energy of a snubber is regenerated as shown in FIG. 3. A PWM cycloconverter is an AC-AC directly power-converting apparatus for directly converting a three-phase AC power source voltage into a variable voltage having variable frequency (arbitrary voltage and frequency). On the basis of the principle of the PWM cycloconverter, pulse current flows in an input unit of the semiconductor device for electric power. In order to prevent the pulse current from returning to a power source system, an AC reactor and an AC capacitor are generally used to provide a low-pass filter. When electric current flows in the filter, however, a resonance phenomenon occurs between the reactor and the capacitor, so that resonance current flows into the input unit. The resonance current deteriorates a rate of distortion of the input current. Further, the electric current flows in the condenser even in the case that the PWM cycloconverter does not operate, so that the resonance current is also generated.
As a conventional method of keeping down the resonance current, proposed is a method of using a small-sized PWM cycloconverter connected to a DC voltage source of a snubber part to perform input current correction.
Patent Reference 1: JP-A-2003-244960